Construction or structural steel belongs to the, by far, largest group of steel grades and is used, among other things, for the construction of ships, buildings, bridges, excavators, agricultural machinery and cranes.
Structural steel is a low-alloy steel. This means that the content of alloying elements is not more than 5%. In general, structural steel meets the following conditions:
- Low carbon content (max. 0.25%)
- Relatively soft material
- Good forming properties
- Good weldability without becoming hard and brittle
- High elongation
Rustproof steel is not a construction steel because of its high chromium content (>10.5%). The alloy content of tool steel is less than 5%. Nevertheless, tool steel is also not a construction steel, as the carbon content is higher than 0.25%.
Classification of structural steels
The terms and variants of structural or engineering steels are defined in the standards EN 10025-2 to 6. The classification of this standard offers a good subdivision of the different steel grades:
- EN 10025-2: Unalloyed structural steels
- EN 10025-3: Normalised / normalised rolled fine grain structural steels
- EN 10025-4: Thermomechanically rolled, weldable and fine grain structural steels
- EN 10025-5: Weather resistant structural steel
- EN 10025-6: Structural steels with high yield strength in the refined state
Structural steel names
The names of structural steel variants start with an "S" (Structural Steel), followed by a number indicating the yield strength (in MPa or N/mm²) of the material. A frequently used structural steel is S355, which has a yield strength of 355 N/mm².
The value of 355 N/mm² in this example indicates the limit above that the material changes from elastic deformation to plastic deformation after being loaded. This value is important for structural engineers regarding the load-bearing capacity for static calculations.
The names of structural steels are fixed and consist of a combination of numbers and letters:
For example: S355J2W+N
- S: Structural steel
- 355: The yield strength in MPA (or N/mm²)
- J: Minimum notch impact energy (J corresponds to 27J)
- 2: The temperature at which the notched bar impact test was performed (2 corresponds to -20°C)
- W: Weather resistant steel
- +N: Normalizing rolled is
There are a large number of other abbreviations that can be used to specify additional properties.
The mechanical values of structural steel largely determine whether a particular material can be used within its application in a structure or workpiece. The most important mechanical properties include
Both the yield strength and the tensile strength decrease with increasing plate thickness. The stated tensile strength is valid for plate thicknesses up to 40 mm.
Attention! Structural steels always have a modulus of elasticity of 210,000 N/mm² (210,000 MPa or 210 Gpa).
|Material||Yield strength||Tensile strength||Notched impact value|
|S235JR||≥ 235 N/mm²||360 – 510 N/mm²||≥ 27 J at 20°C|
|S355J2+N||≥ 355 N/mm²||470 – 630 N/mm²||≥ 27 J at -20°C|
|S420ML||≥ 420 N/mm²||520 – 680 N/mm²||≥ 27 J at -50°C|
|S690QL1||≥ 690 N/mm²||770 – 940 N/mm²||≥ 30 J at -60°C|
Oxidation of structural steel
The iron in structural steel easily reacts with oxygen from the air to form iron oxide. In the common language this is called rusting or corroding. The main disadvantage of steel oxidation is that the surface expands, causing it to burst from the base material and form new openings which then continue to rust. This is in contrast to the surface oxidation of stainless steel, for example, where the chromium oxides form an oxygen-tight layer and protect the underlying material.
The oxidation of structural steel can be prevented, among other things, by providing the surface with an oxygen-tight layer or a metal that sacrifices itself over time before the steel itself begins to oxidise. The most common protective measures are listed below:
Steel products can be easily painted. When used outdoors, it is recommended to paint with a primer and a top coat. The paint can be a 1-component paint (1k) or a 2-component paint (2k), the latter often being harder and more durable. A great advantage of the paint is that it is available in all possible colours.
Powder coating is possible because steel conducts electricity. It means that an electrostatic process is used to apply a layer of powder to the product, which is then melted in the furnace to form a wear-resistant and dense layer. Powder coating can be applied in a variety of colours.
In electrogalvanizing, also known as galvanizing, a thin layer of zinc is applied via an electrolytic process. Via an anode, zinc is put into a bath which is via an electrolytic process deposited as a very thin layer (5 - 40µm) on the product. This process takes place at a maximum temperature of 70°C. This process can be used as a primer for paint or powder coating. Due to the Faraday effect, the inside of a cavity is not or only very limitedly provided with a protective surface.
Hot dip galvanizing
In hot dip galvanizing, the product is dipped into a bath of liquid zinc and in this way coated with a layer of zinc (30 - 200µm). This process takes place at a temperature of about 470°C. In contrast to the surface treatments mentioned above, hot dip galvanizing also protects the inside of a product from oxidation.
When galvanizing is combined with a coating (painting or powder coating), we speak of a duplex system. The degree of protection is increased by up to 250% compared to the addition of the individual treatments.
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